First it must mate successfully with a female four times its size that would prefer to eat it than have its babies. Then, the male must do everything possible to keep eager rivals away from the impregnated female. In the macabre world of spider sex, this means self-emasculation.

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That’s right&colon; coin spiders voluntarily bite off their own genitals. This habit, practised by around 30 spider species, is not the most obvious way to improve sexual performance. But according to Matjaž Kuntner from the Slovenian Academy of Sciences and Arts, eunuchs have an advantage over their intact neighbours.

A close-up of the male – the whitish areas to the left and right are where the the sperm- transferring organs, or palps, used to be

(Image: Matjaz Kuntner)

For one thing, coin spiders only produce enough sperm for a single sexual adventure in their lifetime. So getting rid of the extra baggage – the two sperm-transferring organs known as palps, which can make up around a tenth of their bodyweight – after one use makes them leaner, meaner and better suited to holding off the advances of competing males.

Keeping other males away after mating with a female is particularly important for spiders as several males can fertilise the same batch of eggs. Only by sticking like glue to its mate can a male guarantee that the next generation will carry its genes.

Extreme monogamy

“It is an extreme form of monogamy. Males put all their eggs in one basket and focus on a single female,” Kuntner says.

That is what Kuntner suspected, at any rate. He has previously showed with his collaborators that another species of spider that breaks off its genitals during mating – rather than biting them off afterwards – does it to become a more effective bodyguard. So Kuntner and his team set out to discover if this even more destructive behaviour could have similar benefits.

Individual males were given seven days to mate with a female. The researchers then compared the behaviour of eunuchs with spiders that had never mated.

They found that spiders that were lacking one or both sperm organs after mating were far more feisty than the rival males. The loss of their genitals seemed to give them an extra boost – an arachnid double espresso, if you will.

The eunuchs remained around 50 per cent closer to females and attacked rivals much more aggressively than their virgin competitors. They also stayed active for around 40 per cent longer compared with non-maters when harassed by a researcher’s paintbrush, presumably because they did not have large palps weighing them down. Self-emasculation, it would appear, produces better bodyguards.

Eunuch aggression

Kuntner could not discount the possibility that the act of copulation itself was responsible for giving the spiders a boost – virgin males have little reason to want to protect the female. But he thinks that self-emasculation almost certainly increases the spider’s motivation and aggression. When they only have one chance, they will do whatever it takes to stay ahead.

For the female, this possessive behaviour is actually against her interests, as having multiple mates allows for more varied offspring – which in turn increases the chances of the female’s genes being passed on down generations. But then, she does try to eat the male, so a lasting relationship is hardly the first thing on either spider’s mind.

Kuntner thinks that this very real danger of becoming lunch rather than lover was directly responsible for the evolution of self-emasculation. Coin spiders are much better off minimising their sexual encounters with hungry females, and so a one-off mating strategy becomes the most successful option.

This adaptation in turn drove the limited sperm production and the self-emasculating behaviour – although which of these traits came first is a chicken-and-egg question.